Four speed cameras within a few miles of each other in Surrey have been set alight in the last few months, causing thousands of pounds of damage.
Police suspect the same people may be targeting the cameras
Each time, the offenders have draped old tyres over the bright yellow cameras and poured petrol on them.
The most recent incident was on the A240 Reigate Road, near Banstead, on Wednesday night.
Duncan Knox, of the Surrey Safety Camera Partnership, said the action was irresponsible and could cost lives.
The offenders "are putting people at risk of accidents at these dangerous hotspots... and the possibility that there could be an increase in accidents", the project manager said.
The first incident was last July on the A24 Mickleham bypass, while in December two speed cameras were set alight on the A217 - one at Lower Kingswood and the other on Reigate Hill.
In the latest fire this week, damage to the safety camera was said to have been fairly superficial.
However, damage to the other three was much worse, and they have needed to be replaced.
The cost of the damage depended on whether there was a camera inside the housing.
Mr Knox said if just the basic housing unit was destroyed it could cost between £5,000 and £10,000 to replace.
But if the unit contained a camera then the cost rose to £30,000, he told the BBC News website.
He added that mobile enforcement vehicles were being used at the sites until the cameras were back in working order.
"Police are also looking into the possibility of some sort of surveillance," Mr Knox added.